This invention relates to the field of electrical transducer devices such as are commonly used in the measurement of small physical displacements, forces, vibrations, pressures, and accelerations and to the electronic circuitry used with such devices for signal processing.
In the civil engineering laboratory and in the design of airframes for aircraft, it has become common practice to employ transducers which include sensors such as the resistance strain gauge as measuring tools for evaluating the response of structural members to physical stressing.
Both the piezoelectric crystal and the electrical resistance strain gauge have, in fact, been applied to a great number of measurement situations in which physical phenomena can be characterized by a small change in a physical dimension. In the measurement of fluid pressure, for example, the displacement of a diaphragm is frequently coupled to a piezoelectric crystal sensor or a resistance strain gauge sensor in order that dimensional changes in such an element represent fluid pressure. In a related manner, sensors and transducers of this type are often used to measure the displacement, the displacement change with time (i.e., the velocity) and the change of velocity with time, (i.e., the acceleration), for the moving part.
In such measurements displacement and velocity are, in fact, mathematically related by the first derivative function and displacement and acceleration are similarly related by the second mathematical derivative. Speaking conversely, displacement is in fact mathematically the first integral of velocity and also the second integral of acceleration. These differential and integral relationships are particularly useful in many real world measurement situations where the behavior of a physically moving object is to be fully evaluated from a single transducer signals, as might conveniently occur, for example, in the widely diverse field of machinery designs, explosives development, and vehicle crash testing.
In test situations of this type, it is often convenient, therefore, to employ a single sensor or transducer device and to determine the three quantities of displacement, velocity, and acceleration for the tested element by mathematically operating on the electrical signals obtained from this one device. Provisions for such signal processing are, in fact, provided in the present invention.
According to another aspect of the heretofore practiced measurement systems, it has been common practice to locate one or more sensor or transducer devices in physical contact with a moving object and to couple the resulting signal to a collection of electronic instruments which are located some convenient distance away by using a low-level signal conducting electrical cable. In view of the small amplitude of signals generated by most transducer devices, the quality of the signal received at the distally located signal processing electronic apparatus is often greatly diminished by the presence of signal losses and noise introduced into the low-level signal conductors. An arrangement for improving upon this performance is also provided in the present invention.
The prior patent art discloses a number of systems in which signal data is communicated from a remote location to a processing station. Included in this prior art is the patent of U. V. Helava, U.S. Pat. No. 4,077,030, which teaches a system for transmitting data from a plurality of remote sensors to a digital processor, and the patent of N. F. Douglas, U.S. Pat. No. 4,628,315, which shows the use of an addressable transducer in a monitoring system having a central station and a frequency varying communication scheme between the remote sensor and the digital processor. Also included in this prior art is the U.S. Pat. No. 4,158,765 patent of H. Shauger et al, which discloses an electronic totalizer which is located in a two-wire transmitter that communicates via a current modulated signal with a receiving station, and the U.S. Pat. No. 4,638,451 of R. K. Hester et al, wherein a programmable central processing unit is used to control the receipt of analog data and the characteristics of electrical wave filters operating upon that data. Additionally included in this prior art is the U.S. Pat. No. 3,517,662 patent of H. T. Finch et al, wherein a patient monitoring apparatus adds a patient identifying code to a defective electrocardiogram signal that is received from the remotely-located patient; and the U.S. Pat. No. 4,451,826 patent of G. Fasching, wherein a relatively large number of remote sensor stations communicate with a master station by way of a single transmission line of the coaxial cable type.
Although each of these prior patents includes at least one aspect of similarity with the present invention, neither the individual patents nor their combination is suggestive of a system wherein a relatively large quantity of analog signal processing is disposed remotely and controlled by a central processing apparatus and additional other improvement aspects of the present invention. In this and the description to follow the term transducer is intended to include both a sensor element such as an electrical resistance strain gauge and its associated electrical circuit.